Students Eric Harris and Dylan Klebold opened fire at Columbine High School in Littleton, Colorado, on April 20, 1999. Klebold and Harris killed 12 students and a teacher before killing themselves.

Investigators later learned that Klebold and Harris planned the Columbine massacre and kept journals and video taped recording of their plans.Time Magazine: Top 25 Crimes: Columbine Massacre 1999

Klebold and Harris killed themselves each with a single gunshot following the Columbine massacre on April 20, 1999. Klebold was armed with a 9mm semi-automatic and a sawed off, double-barrel shotgun. Harris was armed with a 9mm carbine and a 12-gauge sawed-off shot gun. Both shooters concealed their weapons upon entering the school. They began planning the attack, which left 15 in total dead (including themselves), a year in advance.Rocky Mountain News: Demonic Plan Was Months in Making (May 20, 2000)